1. Field of Invention
The present invention relates to a surface emitting laser used for digital optical communication, and a method of manufacturing the same.
2. Description of Related Art
A vertical cavity surface emitting laser (VCSEL) generally includes a convex light emitting portion formed by vertically etching semiconductor stacked layers formed by laminating an active layer, and a distributed reflecting layer to emit a laser beam from a light emission plane through an opening formed in the upper surface of the light emitting portion. In this laser, in order to emit a laser beam, upper and lower electrodes are formed above and below the semiconductor stacked layers including the light emitting portion, through insulating layers, and a voltage is applied so that a current flows through, in the thickness direction of the light emitting portion, to supply a current to the active layer.
However, in the surface emitting layer having the above construction, the upper and lower electrodes formed on the upper and lower surfaces, and the insulating thin layers formed between the electrodes, form a capacitor, and thus a lot of time is required for charging and discharging due to the high parasitic capacity of the capacitor. Therefore, the surface emitting layer has difficulty in high-speed modulation.
Thus, for example, a method for forming an insulating thick layer made of a polyimide resin between upper and lower electrodes has been proposed, as disclosed in Japanese Unexamined Patent Application Publication No. 8-116131. In this proposal, a recessed portion that is formed around a projecting portion, serving as a light emitting portion, is filled with a polyimide resin to remove a step formed between the recessed portion and the projecting portion, and an electrode is stacked on the upper surface of the polyimide resin.
In the surface emitting laser having the above construction, the distance between the upper and lower electrodes is increased by the polyimide resin to decrease the parasitic capacity, thereby permitting high-speed modulation in the surface emitting layer.
In order to increase the density of the surface emitting laser having the above construction, electrodes and other semiconductor elements are frequently mounted by flip chip bonding including soldering with solder bumps, or the like.
However, in the surface emitting laser having the above construction, the electrode is stacked on the upper surface of the polyimide resin which is softer than semiconductor materials, thereby possibly causing deformation of the electrode surface, such as depression, during mounting. This results in the electrode becoming broken, or the surface emitting laser not being strongly mounted.
Since a laser beam is emitted via the upper and lower electrodes formed to hold the semiconductor stacked layers therebetween, contact with an electrode at the back of a semiconductor substrate must be achieved by wire bonding or the like.
The present invention addresses the above-described situation, and provides a surface emitting laser permitting secure and strong mounting even in mounting by flip chip bonding, and high-speed modulation, and a method of manufacturing the same.
A surface emitting laser of the present invention includes a semiconductor substrate, semiconductor stacked layers stacked on the substrate and divided into a light emitting portion and a reinforcing portion through a recessed portion, an insulating material buried in the recessed portion, and a pair of electrodes that apply a voltage to pass a current in the thickness direction of the light emitting portion. The pair of electrodes have an external connecting portion formed on the upper surface of the reinforcing portion.
In this construction, the pair of electrodes is formed to have the external connecting portion formed on the upper surface of the reinforcing portion including the semiconductor stacked layers harder than a conventional polyimide resin, and thus deformation, such as recession of the electrode surfaces, can be suppressed even in mounting by flip chip bonding. Therefore, it is possible to address or resolve the problem of peeling of the electrodes, and securely mount the surface emitting laser.
Exemplary embodiments include the following.
The pair of electrodes is formed on the upper surface of the reinforcing portion coplanar with the electrodes, thereby eliminating the need to achieve contact with an electrode on the back by wire bonding or the like. This is effective in decreasing the complexity of mounting of the surface emitting laser.
Furthermore, in the surface emitting laser, one of the pair of electrodes is electrically connected to the lower end of the light emitting portion through a contact hole vertically extending in the insulating material.
Since one of the pair of electrodes is electrically connected to the lower end of the light emitting portion through the contact hole vertically extending in the insulating material, a current can be supplied in the vertical direction, i.e., in the thickness direction, of the light emitting portion, even when the pair of electrodes is formed on the same plane.
The bottom of the recessed portion reaches the surface of the semiconductor substrate over the entire length of the bottom in order to attain non-conduction between the lower end of the light emitting portion and the lower end of the reinforcing portion.
Since the bottom of the recessed portion is formed to reach the surface of the semiconductor substrate over the entire length of the bottom to attain non-conduction between the lower end of the light emitting portion and the lower end of the reinforcing portion, a parasitic capacity produced between the pair of electrodes can be suppressed. Namely, the surface emitting laser can perform higher-speed modulation.
A method of manufacturing a surface emitting laser of the present invention includes vertically etching semiconductor stacked layers formed on a semiconductor substrate to form a recessed portion to divide the semiconductor stacked layers into a light emitting portion and a reinforcing portion, further vertically etching the bottom of the recessed portion until the bottom reaches the surface of the semiconductor substrate over the whole length of the bottom to form a groove to attain non-conduction between the lower end of the light emitting portion and the lower end of the reinforcing portion, filling the recessed portion including the groove with an insulating material, forming a contact hole in the insulating material so that the contact hole vertically extends and connects to the lower end of the light emitting portion, and forming electrodes on the upper surface of the reinforcing portion so that the electrodes are respectively electrically connected to the upper ends of the light emitting portion and the contact hole.
In the step of forming the groove, the groove is formed so that a portion that is connected to the lower end of the contact hole remains at the lower end of the light emitting portion.
Since the groove is formed so that the portion that is connected to the lower end of the contact hole remains at the lower end of the light emitting portion, one of the electrodes can be electrically connected to the lower end of the light emitting portion through the contact hole. Therefore, even when a pair of electrodes is formed on the upper surface of the reinforcing portion, a current can be supplied in the thickness direction of the light emitting portion.
The reinforcing portion includes a portion of the semiconductor stacked layers stacked on the upper surface of the semiconductor substrate, and does not function as the light emitting portion. The portion is removed by etching in manufacturing a conventional surface emitting laser. In the present invention, the reinforcing portion, which has been conventionally removed, is left, and a pair of electrodes is formed on the reinforcing portion to have an external connecting portion.
A photodiode includes a semiconductor substrate, semiconductor stacked layers divided into a light receiving portion and a reinforcing portion by a recessed portion, an insulating material buried in the recessed portion, and a pair of electrodes that detect a current flowing in the thickness direction of the light receiving portion due to incidence of light. The pair of electrodes has an external connecting portion formed on the upper surface of the reinforcing portion.
Since the pair of electrodes is formed on the upper surface of the reinforcing portion including the semiconductor stacked layers harder than conventional polyimide resins so as to have an external connecting portion, deformation, such as recession of the electrode surfaces, can be suppressed even in mounting by flip chip bonding. It is thus possible to address or resolve the problem of peeling of the electrodes, and the like, and securely mount the photodiode.
Since the pair of electrodes is formed on the upper surface of the reinforcing portion coplanar with the electrodes, it is unnecessary to attain contact with an electrode on the back by wire bonding or the like. This is effective in decreasing the complexity of mounting of the photodiode.
In the photodiode, one of the pair of electrodes is electrically connected to the lower end of the light receiving portion through the contact hole vertically extending in the insulating material.
Since one of the pair of electrodes is electrically connected to the lower end of the light receiving portion through the contact hole vertically extending in the insulating material, a current flowing in the vertical direction, i.e., the thickness direction, can be detected, even when the pair of electrodes is formed in the same plane.
The bottom of the recessed portion reaches the surface of the semiconductor substrate over the entire length of the bottom in order to attain non-conduction between the lower end of the light receiving portion and the lower end of the reinforcing portion.
Since the bottom of the recessed portion is formed to reach the surface of the semiconductor substrate over the entire length of the bottom to attain electrical non-conduction between the lower end of the light receiving portion and the lower end of the reinforcing portion, a parasitic capacity between the pair of electrodes can be suppressed. Namely, the bandwidth of the photodiode can be further widened.
A method of manufacturing a photodiode includes vertically etching semiconductor stacked layers formed on a semiconductor substrate to form a recessed portion to divide the semiconductor stacked layers into a light receiving portion and a reinforcing portion, further vertically etching the bottom of the recessed portion until the bottom reaches the surface of the semiconductor substrate over the whole length of the bottom to form a groove to attain non-conduction between the lower end of the light receiving portion and the lower end of the reinforcing portion, filling the recessed portion including the groove with an insulating material, forming a contact hole in the insulating material so that the contact hole vertically extends and connects to the lower end of the light receiving portion, and forming electrodes on the upper surface of the reinforcing portion so that the electrodes are respectively electrically connected to the upper ends of the light receiving portion and the contact hole.
In the step of forming the groove, the groove is formed so that a portion connected to the lower end of the contact hole remains at the lower end of the light receiving portion.
Since the groove is formed so that the portion connected to the lower end of the contact hole remains at the lower end of the light receiving portion, one of the electrodes can be electrically connected to the lower end of the light receiving portion through the contact hole. Therefore, even when a pair of electrodes is formed on the upper surface of the reinforcing portion, a current flowing in the thickness direction of the light receiving portion can be detected.
An optoelectric integrated circuit includes at least an optical waveguide, a mirror for incidence into the optical waveguide, a mirror for emission from the optical waveguide, and electric wiring. The surface emitting laser of the invention, a laser driving circuit that drives the surface emitting laser, the photodiode of the invention, and an amplifier circuit that detects signals from the photodiode are mounted on the electric wiring by flip chip bonding.
Since the surface emitting laser of the invention, the laser driving circuit that drives the surface emitting laser, the photodiode of the invention, and the amplifier circuit that detects signals from the photodiode are mounted on the electric wiring by flip chip bonding, an optoelectric integrated circuit having high reliability can be manufactured.